pair. Tell students that all the seismograms are from the same
earthquake, a quake that occurred on January 14, 1993, with a
magnitude of 3.3, but each was recorded by a different seismograph in
the seismograph network.
2. Project transparencies of one seismogram and the map. Model the
procedure for students as necessary.
3. Give these directions for finding the epicenter of the earthquake
recorded on the five seismograms:
a. On the first seismogram, use the second scale to measure the time-
distance from the nearest 10-second mark to the P wave arrival of the
earthquake. Record the P wave arrival times in the table to the nearest
second.
b. Repeat for the S wave, measuring from the same minute mark.
c. Find the T
s
-T
p
time by subtracting the arrival time of the P wave
from the arrival time of the S wave. Record this time in the table.
d. Use the time/distance table on Master 3.3i to determine the distance
to the epicenter.
e. Repeat this procedure for all of the stations.
f. For each seismogram, draw a circle on the map with the compass,
using the distance you calculated as the radius of the circle. Place the
point of the compass at zero on the map scale and adjust the compass
width to the calculated distance. With the distance set, place the point
of the compass on the station and draw a circle. Mark the outer edge of
each circle with a letter to identify the station.
TEACHING CLUES AND CUES
Emphasize the impor-
tance of accurate mea-
surement, particularly
the need to set the
g. Repeat, setting the compass and drawing circles for 0 five stations.
4. Instruct students to circle the area where all the circles intersect.
Ask: What is this area called? (It is the epicenter of the earthquake.)
C. Conclusion
Build a class discussion around these questions:
Q
What information can be obtained from one seismogram? (The
distance from that seismograph in a 360º circle.)
Q
After the arcs for stations TRYN and FGTN were drawn, where
was the epicenter of this earthquake? Explain. (We don’t know yet.
It could be at either place where the two arcs cross. They are the
common points.)
Q
After all the stations were drawn, where was the actual epicenter of
this earthquake? Where was its focus? (In the area where the arcs
cross just south of station BHT. Directly under the epicenter.)
Q
Why is it necessary to have measurements from at least three
different stations to locate the epicenter of an earthquake? (Answers
will vary but should relate to the above questions.)
Q
Why don’t all of the arcs pass through the same point? (Answers
will vary. Accuracy in measurement and drawing should be two
compass accurately for distance.
Don’t worry if the circles don’t
intersect perfectly, however. Remind
students that an earthquake actually
occurs along a fault, not at one
single point.
A G U
/
F E M A
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E I S M I C
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L E U T H S
